Hipparcos, Gaia, and RVs reveal that the radio emitting F star HD 220242 has an M dwarf companion, a likely source of the radio emission

TL;DR

This study combines astrometric, radial velocity, and spectral data to identify an M dwarf companion to the radio-emitting F star HD 220242, suggesting the companion as the likely source of the observed radio emission.

Contribution

It presents a multi-method approach to identify the stellar companion of HD 220242, linking radio emission to an M dwarf companion, which is a novel case among similar surveys.

Findings

HD 220242 has a stellar companion with a 16.79-year orbit.

The companion is confirmed as an M dwarf star.

Radio emission likely originates from the M dwarf companion.

Abstract

The detection of circularly polarized, low frequency radio emission offers the tantalizing possibility of the observation of interactions between stars and their possible substellar companions, as well as direct emission from exoplanets. Additional follow up of systems with radio emission is key to understanding the true origin of the emission, since multiple astrophysical mechanisms can plausibly lead to such signals. While nineteen M dwarfs were detected by LOFAR in circular polarization as part of the V-LoTSS survey, HD~220242 is the only F star to have a circularly polarized low frequency radio detection in the same survey. We conducted radial velocity follow up with the Habitable-zone Planet Finder and combined these observations with additional archival RVs and \textit{Hipparcos}-\textit{Gaia} proper motion accelerations to determine that HD~220242 has a stellar companion with P=16.79$\pm$0.04\,yrs and a mass of $0.619\pm0.014$\,M$_\odot$. We use Spectral Energy Distribution fitting and lack of any UV excess to rule out a co-evolved white dwarf companion and confirm that the companion is an M dwarf star. Given that F stars lack the coronal properties to produce such coherent emission, and the companion mass and lack of UV excess are consistent with an M dwarf, the radio emission is most plausibly associated with the companion.